A device that requires a high power signal such as a transmitter used in a radio base station uses a power amplifier for signal amplification. However, a high power power amplifier has high power consumption and thus has problems such as heat generation and energy loss. Hence, to improve the efficiency of the amplifier, amplifiers (amplifier apparatuses) using drain modulation such as ET (Envelope Tracking) and EER (Envelope Elimination and Restoration) are developed.
For a modulation scheme for signals to be transmitted by the transmitter, a modulation scheme in which the amplitude of a carrier wave changes over time, such as OFDM (Orthogonal Frequency-Division Multiplexing), is used. In the transmitter that transmits such a signal in which the amplitude of a carrier wave changes by time change, if power that can amplify a transmit signal to its maximum peak value is continuously supplied to the amplifier at all times, then when a transmit signal having the maximum peak value is not required, energy loss or heat generation occurs. However, if supply of insufficient power is performed when a transmit signal having the maximum peak value is required, then the amplifier is saturated, and so forth, and thus signal amplification cannot be appropriately performed. Hence, an amplifier (amplifier apparatus) using drain modulation increases or decreases the power supply power of the amplifier according to a change in envelope component represented by an instantaneous peak value of an input signal, and thereby suppresses wasted power consumption and can perform efficient signal amplification.
Japanese Unexamined Patent Publication No. 2005-236512-A discloses an amplifier in which when an input signal is smaller than a predetermined value, signal amplification is performed by an amplifier for small signals, and when the input signal is larger than the predetermined value, signal amplification is performed by an amplifier that performs drain modulation. According to the amplifier according to the Japanese Unexamined Patent Publication No. 2005-236512-A, when dynamic range is wide and a large signal is amplified, high efficiency is obtained, and when a small signal is amplified, too, signal amplification with very little signal degradation can be performed.